System and Method for Process Monitoring and Intelligent Shut-Off

ABSTRACT

An electrosurgical generator for supplying electrosurgical energy to tissue is disclosed. The generator includes sensor circuitry configured to measure at least one tissue or energy parameter and a controller configured to generate a plot of the at least one tissue or energy parameter including a plurality of tissue parameter values, wherein the controller is further configured to normalize the plot of the at least one tissue or energy parameter with respect to treatment volume.

BACKGROUND

1. Technical Field

The present disclosure relates to electrosurgical apparatuses, systemsand methods. More particularly, the present disclosure is directed toelectrosurgical systems and methods for monitoring electrosurgicalprocedures and intelligent termination thereof based on various sensedtissue parameters.

2. Background of Related Art

Energy-based tissue treatment is well known in the art. Various types ofenergy (e.g., electrical, ohmic, resistive, ultrasonic, microwave,cryogenic, laser, etc.) are applied to tissue to achieve a desiredresult. Electrosurgery involves application of radio frequencyelectrical current to a surgical site to cut, ablate, coagulate or sealtissue. In monopolar electrosurgery, a source or active electrodedelivers radio frequency energy from the electrosurgical generator tothe tissue and a return electrode carries the current back to thegenerator. In monopolar electrosurgery, the source electrode istypically part of the surgical instrument held by the surgeon that isapplied to the tissue. A patient return electrode is placed remotelyfrom the active electrode to carry the current back to the generator.

Ablation is most commonly a monopolar procedure that is particularlyuseful in the field of cancer treatment, where one or more RF ablationneedle electrodes that (usually of elongated cylindrical geometry) areinserted into a living body. A typical form of such needle electrodesincorporates an insulated sheath disposed over an exposed (uninsulated)tip. When the RF energy is provided between the return electrode and theinserted ablation electrode, RF current flows from the needle electrodethrough the body. Typically, the current density is very high near thetip of the needle electrode, which tends to heat and destroy surroundingissue.

In bipolar electrosurgery, one of the electrodes of the hand-heldinstrument functions as the active electrode and the other as the returnelectrode. The return electrode is placed in close proximity to theactive electrode such that an electrical circuit is formed between thetwo electrodes (e.g., electrosurgical forceps). In this manner, theapplied electrical current is limited to the body tissue positionedbetween the electrodes. When the electrodes are sufficiently separatedfrom one another, the electrical circuit is open and thus inadvertentcontact with body tissue with either of the separated electrodesprevents the flow of current.

Bipolar electrosurgical techniques and instruments can be used tocoagulate blood vessels or tissue, e.g., soft tissue structures, such aslung, brain and intestine. A surgeon can either cauterize,coagulate/desiccate and/or simply reduce or slow bleeding, bycontrolling the intensity, frequency and duration of the electrosurgicalenergy applied between the electrodes and through the tissue. In orderto achieve one of these desired surgical effects without causingunwanted charring of tissue at the surgical site or causing collateraldamage to adjacent tissue, e.g., thermal spread, it is necessary tocontrol the output from the electrosurgical generator, e.g., power,waveform, voltage, current, pulse rate, etc.

It is known that measuring the electrical impedance and changes thereofacross the tissue at the surgical site provides a good indication of thestate of desiccation or drying of the tissue, e.g., as the tissue driesor loses moisture, the impedance across the tissue rises. Thisobservation has been utilized in some electrosurgical generators toregulate the electrosurgical power based on measured tissue impedance.

SUMMARY

An electrosurgical generator for supplying electrosurgical energy totissue is disclosed. The generator includes sensor circuitry configuredto measure at least one tissue or energy parameter and a controllerconfigured to generate a plot of the at least one tissue or energyparameter including a plurality of tissue parameter values, wherein thecontroller is further configured to normalize the plot of the at leastone tissue or energy parameter with respect to treatment volume.

According to an embodiment of the present disclosure, a method forsupplying electrosurgical energy to tissue is disclosed. The methodincludes: measuring at least one tissue or energy parameter; generatinga plot of the at least one tissue or energy parameter including aplurality of tissue parameter values; normalizing the plot of the atleast one tissue or energy parameter with respect to treatment volume;and regulating output of the electro surgical generator based on thenormalized plot of the at least one tissue or energy parameter.

A method for supplying electrosurgical energy to tissue is alsocontemplated by the present disclosure. The method includes measuring atleast one tissue or energy parameter; generating a plot of the at leastone tissue or energy parameter including a plurality of tissue parametervalues; filtering the plot of the at least one tissue or energyparameter to form a filtered plot of the at least one tissue or energyparameter; normalizing the filtered plot of the at least one tissue orenergy parameter with respect to treatment volume; and regulating outputof the electrosurgical generator based on the normalized plot of the atleast one tissue or energy parameter.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present disclosure are described herein withreference to the drawings wherein:

FIG. 1A is a schematic block diagram of a monopolar electrosurgicalsystem according to one embodiment of the present disclosure;

FIG. 1B is a schematic block diagram of a bipolar electrosurgical systemaccording to one embodiment of the present disclosure;

FIG. 2 is a schematic block diagram of a generator according to anembodiment of the present disclosure;

FIG. 3 is a plot of treatment volume during application ofelectrosurgical energy according to one embodiment of the presentdisclosure;

FIG. 4 is a plot of treatment volume and impedance during application ofelectrosurgical energy according to one embodiment of the presentdisclosure;

FIG. 5 is a plot of phase angle during application of electrosurgicalenergy according to one embodiment of the present disclosure;

FIG. 6 is a plot of real impedance during application of electrosurgicalenergy according to one embodiment of the present disclosure;

FIG. 7 is a plot of voltage during application of electrosurgical energyaccording to one embodiment of the present disclosure;

FIG. 8 is a plot of average power during application of electrosurgicalenergy according to one embodiment of the present disclosure; and

FIG. 9 is a flow chart of a method according to one embodiment of thepresent disclosure.

DETAILED DESCRIPTION

Particular embodiments of the present disclosure are describedhereinbelow with reference to the accompanying drawings. In thefollowing description, well-known functions or constructions are notdescribed in detail to avoid obscuring the present disclosure inunnecessary detail.

The generator according to the present disclosure can perform monopolarand bipolar electrosurgical procedures as well as microwave ablationprocedures and vessel sealing procedures. The generator may include aplurality of outputs for interfacing with various electrosurgicalinstruments (e.g., a monopolar active electrode, return electrode,bipolar electrosurgical forceps, footswitch, etc.). Further, thegenerator includes electronic circuitry configured for generating radiofrequency power specifically suited for various electrosurgical modes(e.g., cutting, blending, division, etc.) and procedures (e.g.,monopolar, bipolar, vessel sealing).

FIG. 1A is a schematic illustration of a monopolar electrosurgicalsystem according to one embodiment of the present disclosure. The systemincludes an electrosurgical instrument 2 having one or more electrodesfor treating tissue of a patient P. The instrument 2 is a monopolar typeinstrument including one or more active electrodes (e.g.,electrosurgical cutting probe, ablation electrode(s), etc.).Electrosurgical RF energy is supplied to the instrument 2 by a generator20 via an supply line 4, which is connected to an active terminal 30(FIG. 2) of the generator 20, allowing the instrument 2 to coagulate,ablate and/or otherwise treat tissue. The energy is returned to thegenerator 20 through a return electrode 6 via a return line 8 at areturn terminal 32 (FIG. 2) of the generator 20. The active terminal 30and the return terminal 32 are connectors configured to interface withplugs (not explicitly shown) of the instrument 2 and the returnelectrode 6, which are disposed at the ends of the supply line 4 and thereturn line 8, respectively.

The system may include a plurality of return electrodes 6 that arearranged to minimize the chances of tissue damage by maximizing theoverall contact area with the patient P. In addition, the generator 20and the return electrode 6 may be configured for monitoring so-called“tissue-to-patient” contact to insure that sufficient contact existstherebetween to further minimize chances of tissue damage.

FIG. 1B is a schematic illustration of a bipolar electrosurgical systemaccording to the present disclosure. The system includes a bipolarelectrosurgical forceps 10 having one or more electrodes for treatingtissue of a patient P. The electrosurgical forceps 10 includes opposingjaw members having an active electrode 14 and a return electrode 16disposed therein. The active electrode 14 and the return electrode 16are connected to the generator 20 through cable 18, which includes thesupply and return lines 4, 8 coupled to the active and return terminals30, 32, respectively (FIG. 2). The electrosurgical forceps 10 is coupledto the generator 20 at a connector 21 having connections to the activeand return terminals 30 and 32 (e.g., pins) via a plug disposed at theend of the cable 18, wherein the plug includes contacts from the supplyand return lines 4, 8.

The generator 20 includes suitable input controls (e.g., buttons,activators, switches, touch screen, etc.) for controlling the generator20. In addition, the generator 20 may include one or more displayscreens for providing the user with variety of output information (e.g.,intensity settings, treatment complete indicators, etc.). The controlsallow the user to adjust power of the RF energy, waveform parameters(e.g., crest factor, duty cycle, etc.), and other parameters to achievethe desired waveform suitable for a particular task (e.g., coagulating,tissue sealing, intensity setting, etc.). The instrument 2 may alsoinclude a plurality of input controls that may be redundant with certaininput controls of the generator 20. Placing the input controls at theinstrument 2 allows for easier and faster modification of RF energyparameters during the surgical procedure without requiring interactionwith the generator 20.

FIG. 2 shows a schematic block diagram of the generator 20 having acontroller 24, a high voltage DC power supply 27 (“HVPS”) and an RFoutput stage 28. The HVPS 27 is connected to a conventional AC source(e.g., electrical wall outlet) and provides high voltage DC power to anRF output stage 28, which then converts high voltage DC power into RFenergy and delivers the RF energy to the active terminal 30. The energyis returned thereto via the return terminal 32.

In particular, the RF output stage 28 generates sinusoidal waveforms ofhigh RF energy. The RF output stage 28 is configured to generate aplurality of waveforms having various duty cycles, peak voltages, crestfactors, and other suitable parameters. Certain types of waveforms aresuitable for specific electrosurgical modes. For instance, the RF outputstage 28 generates a 100% duty cycle sinusoidal waveform in cut mode,which is best suited for ablating, fusing and dissecting tissue and a1-25% duty cycle waveform in coagulation mode, which is best used forcauterizing tissue to stop bleeding.

The generator 20 may include a plurality of connectors to accommodatevarious types of electrosurgical instruments (e.g., instrument 2,electrosurgical forceps 10, etc.). Further, the generator 20 isconfigured to operate in a variety of modes such as ablation, monopolarand bipolar cutting coagulation, etc. It is envisioned that thegenerator 20 may include a switching mechanism (e.g., relays) to switchthe supply of RF energy between the connectors, such that, for instance,when the instrument 2 is connected to the generator 20, only themonopolar plug receives RF energy.

The controller 24 includes a microprocessor 25 operably connected to amemory 26, which may be volatile type memory (e.g., RAM) and/ornon-volatile type memory (e.g., flash media, disk media, etc.). Themicroprocessor 25 includes an output port that is operably connected tothe HVPS 27 and/or RF output stage 28 allowing the microprocessor 25 tocontrol the output of the generator 20 according to either open and/orclosed control loop schemes. Those skilled in the art will appreciatethat the microprocessor 25 may be substituted by any logic processor(e.g., control circuit) adapted to perform the calculations discussedherein.

A closed loop control scheme is a feedback control loop wherein sensorcircuitry 22, which may include a plurality of sensors measuring avariety of tissue and energy properties (e.g., tissue impedance, tissuetemperature, output current and/or voltage, voltage and current passingthrough the tissue, etc.), provides feedback to the controller 24. Suchsensors are within the purview of those skilled in the art. Thecontroller 24 then signals the HVPS 27 and/or RF output stage 28, whichthen adjust DC and/or RF power supply, respectively. The controller 24also receives input signals from the input controls of the generator 20or the instrument 2. The controller 24 utilizes the input signals toadjust power outputted by the generator 20 and/or performs other controlfunctions thereon.

The present disclosure provides for a system and method for monitoringelectrosurgical procedures using one or more tissue parameters, whichinclude real impedance, imaginary impedance, phase angle, voltage,current, average power and combinations thereof. The use of tissueparameters to control delivery of electrosurgical energy is discussedwith respect to performing ablation procedures, however, those skilledin the art will appreciate that the illustrated embodiments may beutilized with other electrosurgical procedures and/or modes.

In embodiments, various tissue parameters may be measured, recorded andthen plotted to form tissue parameter plots. The tissue parameter plotsare then filtered to obtain a filtered curve that correlates to the sizeof the ablation volume. In particular, FIG. 3 shows a plot 300 of antreatment volume calculated based on temperature measurements. Thetreatment volume may be estimated using an Arrhenius ablation modelimplemented via LABVIEW™ software, which is available from NationalInstruments of Austin, Tex. The software may be executed on a variety ofcomputing devices, such as Luxtron thermal probes available fromLumaSense Technologies of Santa Clara, Calif., that may also interfacewith a plurality of suitable temperature measurement devices thatprovide temperature measurements over a predetermined period of time(e.g., about 15 minutes) to the computing device. The probes may bedisposed at multiple locations to provide for different temperaturemeasurements which allows for extrapolation of the size of the ablationvolume. The software then calculates the estimated treatment volumebased on the models discussed above that are implemented in thesoftware. In embodiments, the size of the ablation volume may also bedetermined by excising the volume, obtaining a plurality of slices ofthe ablation volume and measuring the cross-sectional size of theablation volume of each of the slices. This procedure may be performedto verify the accuracy of the modeled treatment volume as determined bythe Arrhenius ablation model.

FIGS. 4-8 illustrate plots of various tissue parameters. FIG. 4 shows aplot 400 of reactive impedance. Plot 400 is obtained by normalizing aplot of imaginary (e.g., reactive) impedance vs. time. The imaginaryimpedance values are filtered prior to normalization, which may beaccomplished by assigning 1 to an ending value of the plot 400 and 0 tothe starting value. The plot 400 may be smoothed by convolution and thepeaks may then be detected by using an extrema function. Connecting thepeaks of the plot 400 provides for a correlated plot 402, whichsubstantially matches the shape of the treatment volume plot 300. Thismay be accomplished by curve-fitting using a spline function and thencorrelating the two plots 300 and 402. These functions may be performedby using MATLAB™ environment, which provides for convolution, extrema,curve fitting, and correlation functions, available from Mathworks ofNatick, Mass. In particular, the correlation value, ρ, for the plot 402with the plot 300 was about 1, which denotes a high degree ofcorrelation.

Similar correlation is also illustrated by FIGS. 5-8. In particular,FIG. 5 shows a plot 500 of phase angle measurements, which is alsonormalized. Connecting the peaks of the plot 500 provides for acorrelated plot 502, which when inverted substantially matches the shapeof the plot 300. FIGS. 6-8 show a plot 600 of real impedance, a plot 700of voltage, and a plot 800 of average power, respectively, all of whichare scaled. Connecting the peaks of the plot 600 provides for acorrelated plot 602, which when inverted also substantially matching theplot 300. The plots 702 and 802 may be generated based on ripples foundat about the midpoint of the leading rising or falling edge of each ofthe peaks of the plots 700 and 800, respectively. Ripples may beidentified as any fluctuations in the peak aside from the peak itself.The resulting plots are also inverted to provide for the correlatedplots 702 and 802.

The relationship between the plots 402, 502, 602, 702, 802 and 300illustrates the correlation between various tissue parameters, such asreactive impedance, phase angle, real impedance, voltage and averagepower and the size of the ablation as determined using temperaturemeasurements.

Similar to the correlation value of the plot 402 with the plot 300, thecorrelation value for the plots 502 and 602 with the plot 300 was alsoabout 1. This illustrates, that imaginary impedance, real impedance andphase angle yield patterns that are highly correlated with treatmentvolume dynamics and are suitable for detecting process progression andpossible trigger points for initiating procedure termination. Althougheach of the tissue parameters appears to be correlated to the treatmentvolume (i.e., ablation volume), while not wishing to be bound by theory,it is believed that each of the tissue parameters may be measuringdifferent characteristics of tissue consistency.

Complex impedance consists of real and imaginary impedance. Realimpedance is identified with resistance and imaginary impedance isidentified with reactance. In addition, reactive impedance may be eitherinductive or capacitive. Purely resistive impedance exhibits no phaseshift between the voltage and current, whereas reactance induces a phaseshift θ between the voltage and the current passing through the tissue,thus imaginary impedance may be calculated based on the phase angle orphase shift between the voltage and current waveforms.

Changes in the imaginary impedance during energy delivery may be used asan indication of changes in tissue properties due to energy application.More specifically, imaginary impedance may be used to detect theformation of micro bubbles, bubble fields and tissue desiccation thatimpart an electrical reactivity to the tissue that corresponds to sensedimaginary impedance. The tissue reactivity is reflective of the energythat is being delivered into the tissue. Thus, the measured change inimaginary impedance may be used as an indication of the amount of energyresident in the tissue. Monitoring of the resident energy in combinationwith monitoring of the energy being supplied by the generator allows forcalculation of energy escaping the tissue during treatment, therebyallowing for determination of efficiency of the treatment process aswell as any inadvertent energy drains.

As the ablation volume increases, so does the region of tissue that cansupport formation of micro bubbles. The presence of micro bubbles insoft tissue increases the capacitance of the dielectric character of theaffected tissue. As the energy being applied to the tissue increases,the micro bubbles then accrete to form macro bubbles, which decreasesthe capacitance but increases real impedance of the tissue. Hence, theshift of bubble population from micro to macro levels is indicated by ashift of measured impedance from reactive to real. One consequence ofthis is that water content of the tissue is displaced by thistransformation and that displaced water may be harnessed to createdesired tissue effects (e.g., tissue division).

FIG. 9 shows a method for controlling output of the generator 20 basedon various tissue parameters. The method may be embodied as a softwareapplication embedded in the memory 26 and executed by the microprocessor25 to control generator 20 output based on measured tissue parameters orchanges thereof as a function of time. In step 200, ablation energy isdelivered into tissue and various tissue parameters are measured by thesensor circuitry 22. In particular, the sensor circuitry 22 measurestissue and energy parameters based on voltage and current waveformspassing through the tissue and determines voltage, current, averagepower, phase angle between the waveforms, real impedance, and imaginaryimpedance (e.g., the imaginary component of the complex impedance) basedon the phase angle between the waveforms.

In step 202, the tissue and energy parameters are measured and areplotted in real-time to generate a tissue or energy parameter plot asshown in FIGS. 4-8. The plot is pre-filtered to allow for fasterprocessing to generate a pre-filtered curve having smoother curves.Various filters may be utilized to achieve the pre-filtered curve, suchas Kalman Filter and the like. The plot is also normalized as discussedabove, thereafter, the peaks are detected and are interconnected toproduce a correlated plot as shown in FIG. 4. The peaks are detected bythe generator 20 by recording an amplitude value of a specific tissue orenergy parameter (e.g., reactive impedance) corresponding to the peak.The peaks may be identified by tracking the changes in the rate ofchange, e.g., slope of the plotted tissue or energy parameter plot,e.g., plot 400.

In embodiments, as shown in FIGS. 5 and 6, the plot may also be invertedto correlate with the treatment volume plot. In further embodiments, asshown in FIGS. 7 and 8, the plot may be generated based on the ripplesof each of the peaks. The ripples on the rising edge in both voltage andaverage power plots 700 and 800, respectively are due to the energybeing pulsed. The ripples may be detected based on radical changes inthe slope (e.g., rapid oscillations between positive and negative valuesduring a rising edge). The rising or falling edge may also be identifiedby tracking a positive or negative slope, respectively, for apredetermined period of time. The generator 20 then records theamplitude values of the tissue or energy parameter corresponding toripples and generates a plot therethrough. In embodiments, the plot mayalso be inverted.

As the curve is generated, it may be analyzed to determine a shut-offpoint. As discussed above, as energy is applied to the tissue, microbubbles form in the intracellular and intercellular space, resulting ina low starting imaginary impedance (e.g., more negative, associated withmore inductive). As the temperature of the tissue increases, liquidwater is driven away from the tissue regions close to a phase transitiontemperature (e.g., 80° C. and above), more micro bubbles form, steambubbles increase in size and these regions become desiccated. Thedesiccated regions of tissue have higher impedance and thereforecontribute to the capacitive impedance. These phenomena are mostlyreversible because as the temperature increases and drives the waterout, osmotic pressures generate a reverse flow of water. As a result,the tissue seeks a new equilibrium or steady state condition between adesiccated state and a hydrated state to reestablish energy balance.

Once the equilibrium is achieved, the thermal kill zone (e.g., treatmentvolume) does not grow significantly. Thus, establishment of equilibriumcorrelates to the maximum thermal kill zone and may be used to determinewhether termination of energy application is appropriate. In otherwords, monitoring of imaginary impedance allows for determination of theequilibrium, which correlates with the maximum thermal kill zone and maytherefore, serve as a suitable threshold of intelligent shut-off.

Determination of the equilibrium may be determined by analyzing theslope of the tissue parameter curve or the rate of change of theimaginary impedance. The determination of the slope may be performed atthe sensor circuitry 22 and/or the controller 24. A slope of about 0 isbelieved to be reflective of the establishment of equilibrium, whereas anegative slope corresponds to reduction in energy accumulation withinthe tissue. Prior to slope analysis, the tissue parameter curve isfiltered using a single pole recursive filter. Thus, the first filtersmoothes out the impedance curve 110 and the recursive filtering detectsdirection and magnitude of the slope changes as described below.

In step 204, the slope of the tissue parameter curve (e.g., rate ofchange of the tissue parameter) is determined. According to oneembodiment of the present disclosure, the determination of the rate ofchange may be achieved via single pole recursive filtering that averagesa predetermined number of tissue parameter values to achieve the rate ofchange value. Any number of impedance filters may be used and are basedon the following formula (I):

ZfX _(n) =Zin*A+ZfX _(n-1) *B  (1)

A and B are dependent on a time constant and may be specified by theuser, via the input controls of the generator 20, for each particularimpedance filter ZfX. When calculating A and B, the following formulasmay be used:

B=ê(−1/number of samples);

A=1−B.

The sample rate may also be specified by the user for calculating thenumber of samples. In formula (I), Zin is the new root mean squaretissue parameter value (e.g., Zi_(RMS)) just calculated, and ZfX_(n-1)is the filtered tissue parameter, for the filter number specified by X,from the previous iteration through the loop, and ZfX_(n) is the newfiltered impedance value for the filter number specified by X. In oneembodiment, the sample rate for calculating the number of samples may besynchronized with the loop time of the microprocessor 25. Accordingly,within about 5 time constants, the final output of the tissue parameterfilter may be provided that corresponds to the slope of the tissueparameter curve. In another embodiment, an initial base tissue parametermay be used to preload the tissue parameter filters.

In step 206, the slope of the tissue parameter curve is analyzed. In oneembodiment, the slope is analyzed using three regions (e.g., twothresholds). In step 208, it is determined whether the slope is above afirst predetermined threshold (e.g., a positive threshold number). Instep 210 it is determined whether the slope is between the firstthreshold and a second predetermined threshold (e.g., a negativenumber). In step 212, it is determined if the slope is below the secondthreshold. In another embodiments, a plurality of regions may beutilized based on multiple actions that need to be performed in responseto varying slope values. Based on the analysis of the rate of change ofthe tissue parameter (e.g., slope) and/or the tissue parameter, thecontroller 24 adjusts the output of the generator 20 as discussed inmore detail below.

When the slope is above the first threshold, this indicates that thethermal profile is growing and that energy application may continue instep 208. The process then reverts to step 206 to continue slopemonitoring and energy application. When the slope is between the firstand second thresholds, the thermal profile is in equilibrium whichdenotes that equilibrium has been reached and an intelligent shut-offprocess is commenced as shown in step 214. Once it is determined thatequilibrium has been reached, a verification is made if a predeterminedtime delay has expired. This provides a second verification to determinethat a substantial portion of the tissue has been treated. The timedelay may be user-selectable either by entering a predetermined timevalue or by selecting one of proposed delay periods. In one embodiment,one of the options may be a time delay corresponding to the shortesttime for establishing termination of the procedure and another optionmay be a time delay corresponding to a conservative treatment regimenthat assurance 100% cell kill ratio.

In one embodiment, an intermediate time delay may also be utilized. Anintermediate time delay is triggered in step 216 once an equilibrium isreached and the slope detection still continues to make sure that theslope trends do not change. If the slope increases above the firstthreshold, then energy application resumes. At this point, theintermediate time delay is triggered and slope interrogation continues.In other words, the process then reverts to step 206 to continue slopemonitoring and energy application.

When the slope is less than the second threshold, this denotes thatenergy application efficiency is decreasing and the procedure should beterminated. This may be caused by proximity to a blood vessel and otherobstructions. Upon encountering negative slopes that are below thesecond threshold, the process in step 218 terminates the application ofenergy and/or alerts the user of the decrease in energy application.

While several embodiments of the disclosure have been shown in thedrawings and/or discussed herein, it is not intended that the disclosurebe limited thereto, as it is intended that the disclosure be as broad inscope as the art will allow and that the specification be read likewise.Therefore, the above description should not be construed as limiting,but merely as exemplifications of particular embodiments. Those skilledin the art will envision other modifications within the scope and spiritof the claims appended hereto.

1. An electrosurgical generator for supplying electrosurgical energy totissue, comprising: sensor circuitry configured to measure at least onetissue or energy parameter; and a controller configured to generate aplot of the at least one tissue or energy parameter including aplurality of tissue parameter values, wherein the controller is furtherconfigured to normalize the plot of the at least one tissue or energyparameter with respect to treatment volume.
 2. The electrosurgicalgenerator according to claim 1, wherein the at least one tissue orenergy parameter is selected from the group consisting of imaginaryimpedance, real impedance, phase angle, voltage, current and averagepower.
 3. The electrosurgical generator according to claim 1, regulateoutput of the electrosurgical generator based on the normalized plot ofthe at least one tissue or energy parameter.
 4. The electrosurgicalgenerator according to claim 1, wherein the controller is furtherconfigured to filter the plot of the at least one tissue or energyparameter to form a filtered plot of the at least one tissue or energyparameter.
 5. The electrosurgical generator according to claim 4,wherein the controller is further configured to execute at least tworecursive filters configured to recursively process the plot of the atleast one tissue or energy parameter.
 6. The electrosurgical generatoraccording to claim 1, wherein the controller is configured to regulatethe electrosurgical generator based on a rate of change of the at leastone tissue or energy parameter and is configured to control thegenerator to continue application of the electrosurgical energy when therate of change of the at least one tissue or energy parameter is above afirst predetermined threshold.
 7. The electrosurgical generatoraccording to claim 6, wherein the controller is configured to regulatethe electrosurgical generator to discontinue application of theelectrosurgical energy when the rate of change of the at least onetissue or energy parameter is below a second predetermined threshold. 8.The electrosurgical generator according to claim 7, wherein thecontroller is configured to regulate the electrosurgical generator tocommence intelligent shut-off of the electrosurgical energy for aduration of a predetermined time delay when the rate of change of the atleast one tissue or energy parameter is between the first and secondpredetermined thresholds.
 9. The electrosurgical generator according toclaim 8, wherein the controller is configured to regulate theelectrosurgical generator to restart application of the electrosurgicalenergy when the rate of change of the at least one tissue or energyparameter is above the first predetermined threshold during the timedelay.
 10. A method for supplying electrosurgical energy to tissue,comprising the steps of: measuring at least one tissue or energyparameter; generating a plot of the at least one tissue or energyparameter including a plurality of tissue parameter values; normalizingthe plot of the at least one tissue or energy parameter with respect totreatment volume; and regulating output of the electrosurgical generatorbased on the normalized plot of the at least one tissue or energyparameter.
 11. The method according to claim 10, wherein the at leastone tissue or energy parameter is selected from the group consisting ofimaginary impedance, real impedance, phase angle, voltage, current andaverage power.
 12. The method according to claim 10, further comprisingthe step of: recursively processing the plot of the at least one tissueor energy parameter to form a filtered plot of the at least one tissueor energy parameter.
 13. The method according to claim 12, wherein theregulating further includes measuring a rate of change of the imaginaryimpedance of tissue and the regulating step further includes the step ofcontinuing application of the electrosurgical energy when the rate ofchange of the imaginary impedance is above a first predeterminedthreshold.
 14. The method according to claim 13, wherein the regulatingstep further includes the step of: discontinuing application of theelectrosurgical energy when the rate of change of the imaginaryimpedance is below a second predetermined threshold.
 15. The methodaccording to claim 14, wherein the regulating step further includescomprising the step of: commencing intelligent shut-off of theelectrosurgical energy for a duration of predetermined time delay whenthe rate of change of the imaginary impedance is between the first andsecond predetermined thresholds.
 16. The method according to claim 15,wherein the regulating step further includes the step of: restartingapplication of the electrosurgical energy when the rate of change of theimaginary impedance is above the first predetermined threshold duringthe time delay.
 17. A method for supplying electrosurgical energy totissue, comprising the steps of: measuring at least one tissue or energyparameter; generating a plot of the at least one tissue or energyparameter including a plurality of tissue parameter values; filteringthe plot of the at least one tissue or energy parameter to form afiltered plot of the at least one tissue or energy parameter;normalizing the filtered plot of the at least one tissue or energyparameter with respect to treatment volume; and regulating output of theelectrosurgical generator based on the normalized plot of the at leastone tissue or energy parameter.
 18. The method according to claim 17,wherein the at least one tissue or energy parameter is selected from thegroup consisting of imaginary impedance, real impedance, phase angle,voltage, and average power.
 19. The method according to claim 17,wherein the filtering further includes recursively processing the plotof the at least one tissue or energy parameter to form the filtered plotof the at least one tissue or energy parameter.
 20. The method accordingto claim 17, wherein the regulating further includes: measuring a rateof change of the imaginary impedance of tissue and the regulating stepfurther includes the step of continuing application of theelectrosurgical energy when the rate of change of the imaginaryimpedance is above a first predetermined threshold; discontinuingapplication of the electrosurgical energy when the rate of change of theimaginary impedance is below a second predetermined threshold;commencing intelligent shut-off of the electrosurgical energy for aduration of predetermined time delay when the rate of change of theimaginary impedance is between the first and second predeterminedthresholds; and restarting application of the electrosurgical energywhen the rate of change of the imaginary impedance is above the firstpredetermined threshold during the time delay.